With the increasing complexity of machine tools and other automation equipment, it has become more difficult to diagnose problems in their operation. Machine designers have long used the event timing approach to diagnose a machine problem. This approach is based upon machine operating characteristics that include a definite starting time for each operation, and an ending event. In most such machines, each event must be performed within a fixed period of time, with this time period being constant for a given machine. This fixed time is an operating characteristic of the machine, with the machine possessing many such operating characteristics as it carries out various operations. Indeed, each operation may be characterized by a large number of fixed time periods, each having associated therewith a different period of time. Also inherent in the operation of such machines is the occurrence of a starting event, e.g., the initiation of some output, and an ending event such as a signal arising from a limit switch closure.
Because of its adaptability, the PLC has been used to diagnose problems in the operating machine. However, this is not always satisfactory for various reasons. First, this causes PLC ladder logic memory requirements to increase such that some diagnostic routines consume up to 40% of the total ladder logic memory available in the PLC. This often results in the requirement to use more expensive PLCs having larger memories. A second drawback is that PLC scan time is increased. With the machines being controlled becoming more complex and placing additional demands on the PLC, adding diagnostic logic to an already busy PLC exceeds its operating speed, again requiring an upgraded, more expensive PLC. Another problem with incorporating diagnostics in the PLC is that the diagnostic logic tends to clutter up machine control logic, making it more difficult to follow and execute. This frequently results in lengthening the time required to fix some machine problems, resulting in exactly the opposite intent of the diagnostic routine. All three of the aforementioned problems are aggravated when an attempt is made to retrofit an existing machine with a diagnostic routine, as the PLCs in older machines cannot be easily upgraded to larger and faster versions without extensive ladder logic rewrite. In summary, the problems encountered in retrofitting a PLC-controlled machine with a diagnostic program make this alternative impractical.
Several approaches to removing the diagnostics from the PLC have been attempted. One approach employs a system that reads the machine's input/output (I/O) points as fast as the PLC itself accesses this information. This approach can be used only with a limited number of the larger and more expensive PLCs and is itself very expensive. Another approach allows the machine designer to specify to valid states of the machine. This approach is satisfactory so long as all valid machine states were correctly input. However, only one intimately familiar with this state table can work on this system, making field modifications impractical. Retrofitting of this type of system also requires a machine expert generally in the form of an expensive consultant to program the system.
The present invention overcomes the aforementioned limitations of the prior art by providing diagnostics for a PLC operated system in a manner which is completely passive and invisible to the PLC and which does not therefore place requirements on PLC memory capacity or restrict PLC operating speed. The PLC program may thus be entirely devoted to control functions and the stand alone diagnostic program is more easily maintained .